A large number of chemical compounds are currently derived from petrochemicals. Alkenes (such as ethylene, propylene, the different butenes, or else the pentenes, for example) are used in the plastics industry, for example for producing polypropylene or polyethylene, and in other areas of the chemical industry and that of fuels.
Ethylene, the simplest alkene, lies at the heart of industrial organic chemistry: it is the most widely produced organic compound in the world. It is used in particular to produce polyethylene, a major plastic. Ethylene can also be converted to many industrially useful products by reaction (of oxidation, of halogenation).
Propylene holds a similarly important role: its polymerization results in a plastic material, polypropylene. The technical properties of this product in terms of resistance, density, solidity, deformability, and transparency are unequalled. The worldwide market for polypropylene has grown continuously since its invention in 1954.
Butylene exists in four forms, one of which, isobutylene, enters into the composition of methyl-tert-butyl-ether (MTBE), an anti-knock additive for automobile fuel. Isobutylene can also be used to produce isooctene, which in turn can be reduced to isooctane (2,2,4-trimethylpentane); the very high combustion/explosion ratio of isooctane makes it the best fuel for so-called “gasoline” engines. Amylene, hexene and heptene exist in many forms according to the position and configuration of the double bond. These products have real industrial applications but are less important than ethylene, propylene or butenes.
All these alkenes are currently produced by catalytic cracking of petroleum products (or by a derivative of the Fisher-Tropsch process in the case of hexene, from coal or gas). Their cost is therefore naturally indexed to the price of oil. Moreover, catalytic cracking is sometimes associated with considerable technical difficulties which increase process complexity and production costs.
Independently of the above considerations, the bioproduction of plastics (“bioplastics”) is a thriving field. This boom is driven by economic concerns linked to the price of oil, and by environmental considerations that are both global (carbon-neutral products) and local (waste management).
The main family of bioplastics is that of the polyhydroxyalkanoates (PHA). These are polymers obtained by condensation of molecules comprising both an acid group and an alcohol group. Condensation takes place by esterification of the acid on the alcohol of the following monomer. This ester bond is not as stable as the direct carbon-carbon bond present in the polymers of conventional plastics, which explains why PHAs have a biodegradability of a few weeks to a few months.
The PHA family includes in particular poly-3-hydroxybutyrate (PHB), a polymer of 3-hydroxybutyrate, and polyhydroxybutyrate-valerate (PHBV), an alternating polymer of 3-hydroxybutyrate and 3-hydroxyvalerate.
PHB is naturally produced by some strains of bacteria such as Alcaligenes eutrophus and Bacillus megaterium. Laboratory bacteria, like E. coli, having integrated synthetic pathways leading to PHB or to PHAs in general, have been constructed. The compound or its polymer can, in certain laboratory conditions, account for up to 80% of the bacterial mass (Wong M S et al., Biotech. Bioeng., 2008). Industrial-scale production of PHB was attempted in the 1980s, but the costs of producing the compound by fermentation were considered too high at the time. Projects involving the direct production of these compounds in genetically modified plants (having integrated the key enzymes of the PHB synthetic pathway present in producer bacteria) are in progress and might entail lower operating costs.
The production by a biological pathway of alkanes or other organic molecules that can be used as fuels or as precursors of synthetic resins is called for in the context of a sustainable industrial operation in harmony with geochemical cycles. The first generation of biofuels consisted in the fermentative production of ethanol, as fermentation and distillation processes already existed in the food processing industry. The production of second generation biofuels is in an exploratory phase, encompassing in particular the production of long chain alcohols (butanol and pentanol), terpenes, linear alkanes and fatty acids. Two recent reviews provide a general overview of research in this field: Ladygina N et al., Process Biochemistry, 2006, 41:1001; and Wackett LP, Current Opinions in Chemical Biology, 2008, 21:187.
In the alkene chemical family, isoprene (2-methyl-1,3-butadiene) is the terpene motif which, through polymerization, leads to rubber. Other terpenes might be developed, by chemical, biological or mixed pathway, as usable products such as biofuels or to manufacture plastics. The recent literature shows that the mevalonate pathway (a key intermediate in steroid biosynthesis in many organisms) might be used in order to efficiently produce products from the terpene family at industrial yields (Withers S T et al., Appl. Environ. Microbiol., 2007, 73:6277).
The production of terminal alkenes [ethylene mono- or di-substituted at position 2: H2C═C(R1)(R2)] has apparently been less extensively investigated. The production of isobutylene from isovalerate by the yeast Rhodotorula minuta has been detected (Fujii T. et al., Appl. Environ. Microbiol., 1988, 54:583), but the efficiency of this conversion, less than 1 millionth per minute, or about 1 for 1000 per day, is far from permitting an industrial application. The reaction mechanism was elucidated by Fukuda H. et al. (BBRC, 1994, 201(2):516) and involves a cytochrome P450 enzyme which decarboxylates isovalerate by reduction of an oxoferryl group FeV═O. At no point does the reaction involve hydroxylation of isovalerate. Isovalerate is also an intermediate in leucine catabolism. Large-scale biosynthesis of isobutylene by this pathway seems highly unfavorable, since it would require the synthesis and degradation of one molecule of leucine to form one molecule of isobutylene. Also, the enzyme catalyzing the reaction uses heme as cofactor, poorly lending itself to recombinant expression in bacteria and to improvement of enzyme parameters. For all these reasons, it appears very unlikely that this pathway of the prior art can serve as a basis for industrial exploitation. Other microorganisms have been described as being marginally capable of naturally producing isobutylene from isovalerate; the yields obtained are even lower than those obtained with Rhodotorula minuta (Fukuda H. et al, Agric. Biol. Chem., 1984, 48:1679).
These same studies have also described the natural production of propylene: many microorganisms are capable of producing propylene, once again with an extremely low yield.
The production of ethylene by plants has long been known (Meigh et al, 1960, Nature, 186:902). According to the metabolic pathway elucidated, methionine is the precursor of ethylene (Adams and Yang, PNAS, 1979, 76:170). Conversion of 2-oxoglutarate has also been described (Ladygina N. et al., Process Biochemistry 2006, 41:1001). Since a single ethylene molecule requires the previous production of a four- or five-carbon chain, the equipment and energy needs of all these pathways are unfavorable and do not bode well for their industrial application for alkene bioproduction.
Prior to the characterization of the enzymatic steps which, in plants, convert to ethylene its true metabolic precursor, S-adenosylmethionine (SAM) via formation of 1-amino-cyclopropane-1-carboxylate (ACC) (Adams and Yang, PNAS, 1979, 76:170), several other hypotheses had been proposed in the scientific literature to explain ethylene production, among which was the decarboxylation of acrylate (H2C═CH—CO2H) originating from the dehydration of 3-hydroxyproprionate. Several articles specifically speculated on the metabolic pathway which would convert 3-hydroxypropionate to ethylene, via acrylate, in order to interpret radiotracer studies of ethylene production in which 14C-labelled substrates were supplied to plant tissue preparations: beta-alanine-2−14C to bean cotyledon extracts (Stinson and Spencer, Plant Physiol., 1969, 44:1217; Thompson and Spencer, Nature, 1966, 210:5036), and propionate-2−14C to banana pulp homogenates (Shimokawa and Kasai, Agr. Biol. Chem., 1970, 34(11):1640). All these hypotheses of the involvement of 3-hydroxypropionate and acrylate in metabolic ethylene production, which did not lead to characterization of enzyme activities, vanished from the scientific literature once the role of methionine, SAM and ACC was discovered (Hanson and Kende, Plant Physiology, 1976, 57:528; Adams and Yang, PNAS, 1979, 76:170).
Therefore, to my knowledge, there is currently no efficient method for producing terminal alkenes such as ethylene, propylene, 1-butylene, isobutylene, 1-amylene or isoamylene by microbiological synthesis. Such method would make it possible to avoid the use of petroleum products, and to lower the costs of producing plastics and fuels. Finally, it could potentially have a considerable global environmental impact by allowing carbon to be stored in solid form.